Advanced Image Coding

AIC - Algorithm Details

These pages describe the algorithms used by the Advanced Image Coding codec.
Note that these algorithms are not new to this codec. Existing algorithms from
the H.264 and JPEG standards are combined in the AIC codec, although usually
modified a bit to fit the needs of the codec.

The following figures show the process flow of the AIC encoder and decoder:

AIC Encoder

AIC Decoder

Both encoder and decoder share the same functional elements:

Color Conversion.
A source RGB image is transformed to the YCbCr color space that has less entropy
in the chominance channels (Cb and Cr) and thus makes these channels better
compressible. This only applies to color images. Grayscale images have only
1 channel.

Block Prediction.
Every channel (Y, Cb and Cr for color images, or Y for grayscale images) is
split into blocks of 8x8 pixels. The contents of each block is predicted from
previously encoded and decoded blocks in the channel. There are 9 ways to
predict the current block. The prediction mode that minimizes the differences
between the original and predicted block is chosen. To save bits on coding
the prediction mode, the blocks in the Cb and Cr channels use the same prediction
modes as the corresponding blocks in the Y channel.
The values in the predicted block are subtracted from the ones in the original
block to form a residual block. When a good prediction mode is chosen, the
values in this residual block are smaller than the original pixel values and
thus better compressible.

Discrete Cosine Transform (DCT) and Quantisation.
To further reduce the entropy of the residual block, this block is transformed
using the Discrete Cosine Transform. The resulting DCT coefficients are usually
smaller than the original values. By quantising these coefficients with a
certain value based on a chosen quality level, the coefficients are reduced
even further. As a result, many of the coefficients in the block will be zero,
and most others will be close to zero.

Context Adaptive Binary Arithmetic Coding (CABAC).
The prediction mode and quantised DCT coefficients are finally encoded to
the stream using a binary arithmetic coding scheme that adapts to the context
in which these values are coded. This means that commonly used prediction
modes and DCT coefficients are coded using fewer bits than rare prediction
modes and coefficients.